The Cell Cycle Introduction to Biology The Key

The Cell Cycle Introduction to Biology

The Key Roles of Cell Division • The ability to reproduce is one of the key features that separates life from non-life. • All cells have the ability to reproduce, by making exact copies of themselves.

• In unicellular organisms, division of one cell reproduces the entire organism • In multicellular organisms, cell division is needed for: o Development of an embryo from a sperm/egg o Growth o Repair

LE 12 -2 100 µm Reproduction 200 µm Growth and development 20 µm Tissue renewal

Asexual Reproduction • Asexual reproduction is reproduction that involves a single parent producing an offspring. o The offspring produced are, in most cases, genetically identical to the single cell that produced them. o Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring. o Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges)

Sexual Reproduction • In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. o The offspring produced inherit some genetic information from both parents. o Most animals and plants, and many single-celled organisms, reproduce sexually.

Contrasting Reproduction Types

Cell Division • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA. • A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

Cellular Organization of the Genetic Material • A cell’s endowment of DNA (its genetic information) is called its genome. • DNA molecules in a cell are packaged into chromosomes.

Chromosomes • The genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • Every cell must copy its genetic information before cell division begins. • Each daughter cell gets its own copy of that genetic information. • Cells of every organism have a specific number of chromosomes.

Prokaryotic Chromosomes • Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm. • Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

Eukaryotic Chromosomes • In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

• Chromatin is composed of DNA and histone proteins.

• DNA coils around histone proteins to form nucleosomes.

• The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

Chromosomes During Cell Division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

LE 12 -4 0. 5 µm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromeres Sister chromatids

Phases of the Cell Cycle • The cell cycle consists of o Mitotic (M) phase (mitosis and cytokinesis) o Interphase (cell growth and copying of chromosomes in preparation for cell division) • Interphase (about 90% of the cell cycle) can be divided into subphases: o G 1 phase (“first gap”) o S phase (“synthesis”) o G 2 phase (“second gap”)

LE 12 -5 INTERPHASE S (DNA synthesis) G 1 M MIT (M) OTIC PH AS E ito t y C si in k o s is s e G 2

G 1 Phase: Cell Growth • In the G 1 phase, cells increase in size and synthesize new proteins and organelles.

S Phase: DNA Replication • In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

G 2 Phase: Preparing for Cell Division • In the G 2 phase, many of the organelles and molecules required for cell division are produced.

M Phase: Cell Division • In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. o Mitosis is the division of the cell nucleus. o Cytokinesis is the division of the cytoplasm.

Important Cell Structures Involved in Mitosis • Chromatid – each strand of a duplicated chromosome • Centromere – the area where each pair of chromatids is joined • Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle • Spindle – long proteins (part of the cytoskeleton) that the centrioles produce o Helps move the chromosomes into place.

Prophase • During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible.

Prophase • The centrioles move to opposite sides of nucleus and help organize the spindle.

Prophase • The spindle forms and DNA strands attach at a point called their centromere.

Prophase • The nucleolus disappears and nuclear envelope breaks down.

Metaphase • During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

Metaphase • The spindle fibers connect the centromere of each chromosome to the two poles of the spindle.

Anaphase • During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

Anaphase • The chromosomes separate into two groups near the poles of the spindle.

Telophase • During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

Telophase • A nuclear envelope reforms around each cluster of chromosomes.

Telophase • The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus.

Cytokinesis • Cytokinesis is the division of the cytoplasm. • The process of cytokinesis is different in animal and plant cells.

Cytokinesis in Animal Cells • The cell membrane is drawn in until the cytoplasm is pinched into two equal parts. • Each part contains its own nucleus and organelles.

LE 12 -9 a 100 µm Cleavage furrow Contractile ring of microfilaments Daughter cells Cleavage of an animal cell (SEM)

Cytokinesis in Animal Cells • In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall. • Instead, a cell plate forms between the divided nuclei that develops into cell membranes. • A cell wall then forms in between the two new membranes.

LE 12 -9 b Vesicles forming cell plate Wall of parent cell Cell plate 1 µm New cell wall Daughter cells Cell plate formation in a plant cell (TEM)

LE 12 -10 Nucleus Nucleolus Chromatin condensing Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Chromosomes Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Cell plate Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore microtubules shorten. 10 µm Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

LE 12 -6 ca INTERPHASE PROMETAPHASE

10 µm LE 12 -6 da METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Virtual Onion Root Tip Mitosis Lab Click here to start

Binary Fission • Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission • In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

LE 12 -11_1 Cell wall Origin of replication Plasma membrane E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Two copies of origin Bacterial chromosome

LE 12 -11_2 Cell wall Origin of replication Plasma membrane E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Bacterial chromosome Two copies of origin Origin

LE 12 -11_3 Cell wall Origin of replication E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Two copies of origin Origin Plasma membrane Bacterial chromosome Origin

The Evolution of Mitosis • Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission • Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

LE 12 -12 Bacterial chromosome Prokaryotes Chromosomes Microtubules Intact nuclear envelope Dinoflagellates (Type of plankton) Kinetochore microtubules Intact nuclear envelope Diatoms (Type of Algae) Kinetochore microtubules Centrosome Most eukaryotes Fragments of nuclear envelope

The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

LE 12 -14 G 1 checkpoint Control system G 1 M M checkpoint G 2 S

• For many cells, the G 1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G 1 checkpoint, it will usually complete the S, G 2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G 0 phase

LE 12 -15 G 0 G 1 checkpoint G 1 If a cell receives a go-ahead signal at the G 1 checkpoint, the cell continues on in the cell cycle. G 1 If a cell does not receive a go-ahead signal at the G 1 checkpoint, the cell exits the cell cycle and goes into G 0, a nondividing state.

• An example of external signals is densitydependent inhibition, in which crowded cells stop dividing • Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum (connective tissue) in order to divide

LE 12 -18 a Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). Normal mammalian cells 25 µm

LE 12 -18 b Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. 25 µm Cancer cells

Loss of Cell Cycle Controls in Cancer Cells • Cancer cells do not respond normally to the body’s control mechanisms • Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue • If abnormal cells remain at the original site, the lump is called a benign tumor • Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

LE 12 -19 Lymph vessel Tumor Blood vessel Glandular tissue Cancer cell A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. Metastatic tumor A small percentage of cancer cells may survive and establish a new tumor in another part of the body.